It is frequently desired to process an input video signal at a time when the continued presence of the video signal is critical, e.g., when the signal is being broadcast or otherwise transmitted to television receivers. This processing may involve use of, for example, a frame synchronizer, a processing amplifier or a video corrector. Conventionally, the processing device is incorporated in an apparatus having two signal channels between its input terminal and its primary, or program, output terminal, namely a processing channel, in which the processing is effected, and a bypass channel, whereby the input video signal is passed to the program output without modification. The processing channel includes the processing device itself. If the processing device is not functioning correctly at the time of a switch from the bypass channel to the processing channel, the output signal from the processing apparatus may be garbled, and accordingly the television picture may be momentarily degraded. In the case where the processing device is a frame synchronizer including a phase lock system for locking to the burst and sync information of the input video signal, the degradation of the picture may be alleviated, or the period of degradation reduced, by using a phase lock system having a short response time. However, the lock-up time cannot be reduced arbitrarily without regard for the operation of the system under normal conditions.
It is known to connect a high impedance tap to a video signal channel to enable the signal to be monitored without disturbing the signal passed by the channel. This so-called loop-through arrangement is not used with a processing apparatus that has both an operating channel and a bypass channel.